One benefit to complete presentations is that because everyone's personal tastes differ, some people favor cues that others don't and so on, and because of this some cues heard in the film may have compelled a listener to pick up an album in the first place, only to find the deal-maker cue they enjoyed on screen wasn't included on album at all.
Take the Independence Day album - the original RCA Victor album.
As a soundtrack album it's too one-sided. On one hand it was appropriate for its time, because it represented the large-scale action and suspense music for almost all its 50+ minute running time, and the film was primarily seen for its visual effect sequences.
To add to that, I'm sure the big and loud bombast is what's so appealing for a lot of film music fans.
On the other hand, because it's so one-sided, it all (save for very minor exceptions in "El Toro Destroyed" and "Aftermath") ignored the score's softer, tender, more emotional side. That alone could have filled a 50 minute CD on its own. That was the music that stuck out for me, and when I picked it up (it was my very first CD to go with my first CD player) I was frustrated by the lack of that aspect of the score represented. The First Lady's death, Wedding and farewell/pre-launch of the alien attacker were all representative of that tender side that I had missed on that album.
In that sense I'm grateful for the upcoming complete release, because it's a truly balanced presentation of both personalities of the score.
As someone who loves to see, as opposed to hear, how music works when matched to picture, having alternate cues makes for a great study. You sort of get into the composer's mind and can note what changes were made with regards to the overall story, and if you have the means you can see for yourself how that alternate approach would have influenced the final result - see why it was changed.
Here's an example from Independence Day I put up a long while ago:
http://www.youtube.com/watch?v=vOwa9eZgZpU
It becomes fairly obvious that this piece of music was not only too big, but foreshadowed too much. Its in-film replacement was far superior.
I must say something about "The Perfect Storm".
I love that album. Completely.
However, there's a whole other album of music in the film that didn't make the CD which would make for a great companion disc, and it wouldn't even have to undo the album as it currently exists. There was just such a strong presence of music in that film and what didn't make the CD was as compelling as what did. In the case of some of the action cues, maybe even a little more: That pararescue chopper arrival sequence, where they attempt to save the sailboat passengers was a wonderfully adventurous, high-sprited piece and had a great take on the theme used throughout the cue "Small Victories". The eventual chopper ditch into the ocean sequence has a very elaborate string /anvil piece that is fantastic and fast-paced.